An electric arc furnace is an electrically operated furnace used for melting metal and/or for cleaning slag. The operation of the furnace is based on an arc flame that burns either between separate electrodes, or between electrodes and the material to be melted. The furnace may be operated either by AC or DC cur-rent. Heat is created in the arc flame, and also in the material to be melted, in the case where the arc flame burns between the material and the electrodes. Electric power is conducted to vertical electrodes that are usually located symmetrically in a triangle with respect to the midpoint of the furnace. In the case of a DC smelting furnace there is one electrode in the middle of the furnace. The assembly depth of the electrodes in the furnace is continuously adjusted, because they are worn at the tips owing to the arc flame.
A Söderberg-type electrode of an electric arc furnace is a vertical column comprising a steel casing sur-rounding and covering the electrode paste formed of a graphite-based material. The electrode column is continuously filled with the electrode paste which is introduced from above into the steel casing. The paste is subject to different conditions along the column making it evolve through different phases, ranging from raw paste in the upper part of the steel casing to melted paste in the area starting above the contact shoe ring and further to baked paste in the lower part of the electrode column below the contact shoe ring.
In addition to the contact shoe ring the lower part of the electrode column assembly comprises a pressure ring and a heat shield. The contact shoe ring consists of a plurality of contact shoe elements arranged as a ring to be in contact with a steel casing inside of which the electrode paste is sintered. The contact shoe elements conduct electric current to the electrode. A pressure ring is arranged on the outside of the contact shoe ring, so that the contact shoe ring is surrounded by said pressure ring. The pressure ring consists of a plurality of pressure blocks connected with each other as a ring pressing the contact shoes against the steel casing of the electrode. A heat shield surrounding the electrode column assembly is arranged above the pressure ring in the axial direction of the electrode column assembly. Also the heat shield is comprised of a plurality of segments connected with each other to form an assembly of annular form.
So, because the furnace must be operational continuously and uninterruptedly electrode paste must continuously be introduced into the steel casing. Therefore, one must all the time be aware of the height of the paste column, i.e. of the level of paste in the vertical direction in order to know when and how much paste must further be introduced into the steel casing. Further, because the state of paste is transformed along the height of the paste column from raw paste to softened or melted paste and further to baked paste it is important to know on which level the surface of the melted paste each time exists. This information is used e.g. in the control of the process. Excessive soft paste levels as well as inadequate soft paste levels cause different detrimental effects on the operation of the furnace.
Different methods and equipment have been used for determination of the length and/or state of the electrodes in electric-arc furnaces. Nowadays the determination and measurement of the surface levels of the paste column is normally carried out manually with a wire or tape as measuring instrument. Manual measurement and determination is not always exact enough and further it is sometimes quite difficult to perform due to the extreme environmental circumstances.
As examples of other prior art methods and equipment reference is made to publication EP1209243A2 disclosing a multifrequency equipment for sensing the state of the electrodes in electric-arc furnaces. Publication WO2004/028213A1 discloses an electrode column and a method of determining the length of the electrode in said column in an active furnace. The column is a Söderberg column including a mantel in which the electrode is movable in an axial direction by movable slipping clamps. Publication US2013/0127653A1 discloses a device and an apparatus for measuring the length of an electrode or determining the position of a consumable cross-section of the electrode in an electric furnace, in which the measuring is performed by radar. Publication U.S. Pat. No. 4,761,892 discloses an apparatus for measuring the length of the electrodes in an electric furnace, wherein the measurement is performed by a measuring rod inserted into the furnace.